The present invention relates to a steering box for a motor vehicle, of the type comprising a rotatable toothed pinion and a rack meshing with the pinion and movable axially within a casing, by means of which it is possible to effect displacements of the rack itself with reduced friction, for the purpose of making the steering of the motor vehicle light and precise.
As is known, in steering boxes of the rack and pinion type, the toothed pinion receives the drive from the steering column while the rack controls the steering linkage to the wheels. The rack is usually supported at one end by a sliding bushing, and at the region where the pinion and the rack teeth mesh, by a sliding block provided with a normally cylindrical support surface which can couple with the corresponding surface of the rack opposite the teeth. The coupled surface carries the force which is transferred between the teeth of the pinion and the rack itself.
Steering boxes of this type have the disadvantage of requiring rather high forces on the steering wheel to effect steering, and in particular for movements of the vehicle from rest or in certain operating conditions. This is due to the high forces of rubbing friction which arises from the engagement between the said support surface and the corresponding contact surface of the rack, which forces oppose the axial displacement thereof.
It has also been proposed to support one end of the rack, instead of with a normal sliding bushing, with bushings of the recirculating ball type. The improvements which are obtained with the adoption of such constructional arrangements are very modest. Although the friction between the bushing and the rack is significantly reduced, the greater magnitude of friction between the sliding block and the rack remains unchanged. In this latter region, significant friction occurs because of the very high pressures which are transferred to the contacting surfaces of the rack and sliding block.